Turning to FIG. 1A of the drawings, a conventional branch-line hybrid coupler 100 can be seen. Typically, coupler 100 has four ports and four transmission lines 102-1 through 102-4. Each of these four transmission lines is a one-quarter wavelength transmission line (for a given target frequency). Port 1 is often referred to as an “input port,” which receives an input signal, while port 2 and 3 are referred to as “output ports.” Because the of geometry of coupler 100, the output signal at port 2 is 90° out-of-phase from the input signal, while the output signal at port 3 is 180° out-of-phase from the input signal. This arrangement also allows the power to be split evenly between ports 2 and 3. Port 4 is often referred to as an “isolation port” and is usually terminated with no theoretical loss; an example of such an arrangement can be seen in FIG. 2 of U.S. Pat. No. 5,410,734.
The coupler 100, however, is sensitive to line loss. As it can be observed in FIGS. 1B and 1C, the 90° bandwidth decreases as attenuation increases. In particular, in FIG. 1B, at about 160 GHz, the phase difference between ports 2 and 3 is about 90°, and in FIG. 1C the attenuation for ports 2 and 3 or power difference between ports 2 and 3, and port 1 at about 160 GHz is about −4 dB. Moreover, the gain matching is fine, with more loss. Therefore, there is a need for an improved mixer.
Another conventional circuit that uses hybrid couplers is Lim et al., “A Balanced Power Amplifier Utilizing the Reflected Input Power,” 2009 IEEE International Symposium on Radio-Frequency Integration Technology, pgs. 88-91.